Adaptive Wavefront Control using a Nonlinear Zernike Filter

A conventional Zernike filter measures wavefront phase by superimposing the aberrated input beam with a phase-shifted version of its zero-orderspectral component. The Fourier-domain phase-shifting is performed by afixed phase-shifting dot on a glass slide in the focal plane of a Fourier-transforming lens. Using an optically-controlled phase spatiallight modulator (SLM) instead of the fixed phase-shifting dot, we havesimulated and experimentally demonstrated a nonlinear Zernike filterrobust to wavefront tilt misalignments. In the experiments, a liquid-crystal light valve (LCLV) was used as the phase SLM. The terminology "nonlinear" Zernike filter refers to the nonlinear filteringoperation that takes place in the Fourier domain due to the phase changefor field spectral components being proportional to the spectral componentintensities. Because the Zernike filter output intensity is directlyrelated to input wavefront phase, a parallel, distributed feedback systemcan replace the wavefront reconstruction calculations normally requiredin adaptive-optic phase correction systems. Applications include high-resolution phase distortion suppression for atmospheric turbulence,optical phase microscopy, and compensation of aberrations in optical system components. A factor of eight improvement in Strehl ratio wasobtained experimentally, and simulation results suggest that even betterperformance could be obtained by replacing the LCLV with a more sophisticated optically-controlled phase SLM.SPIE Proc., High-Resolution Wavefront Control:Methods, Devices, and Applications II, Vol. 4124, pp. 198-200, 2000.